Yeast and yeast metabolites are widely used in an array of food and feed products. Baker's and brewer's yeast, for example, are excellent sources of nutrients and flavoring agents. Nutrients that are obtainable from cells include insoluble and soluble cell wall polysaccharides, oligosaccharides, glucans, proteins, peptides, nucleotides, and the like. Cells, in particular cell walls, are also thought to absorb pathogens and consequently to provide a measure of prophylaxis against infection.
Live cells, whole lysed cells, and cell fractions are of particular value in feed and pet food formulations. Lysed cells and cell fractions are thought to contain many nutritive components in a form that is bio-available to the consuming animal. Live yeast cells are thought to aid in digestion in ways not fully understood at present. Whole dead cells, on the other hand, are not thought to be of particular nutritive benefit, except possibly in ruminant animals. The digestive tract of monogastric animals is essentially unable to rupture the cell wall, and thus the majority of the dead cells pass through the digestive tract and are typically excreted whole, without releasing nutrients to the animal.
Consequently, if it is desired to obtain nutrients from dead yeast cells, generally it is necessary to rupture the walls of the cells to allow release of the nutrients. A number of methods are known for rupturing yeast cells, these including mechanical, hydrolytic and autolytic methods. Mechanical methods typically are employed in small-scale laboratory applications. Conventional mechanical disruption includes presses, such as the French press; homogenizers; sonic disruptors, and so forth. In a laboratory French press, for example, pressures as high 20,000 psi and high shear conditions are produced by passing the cells through a small orifice. Other devices subject the cell to different stresses but provide the same result, that is, rupture of the cell wall. For instance, another known apparatus, the bead beater, contains ceramic or glass pellets that are used to crush, shear and fracture cells. Hydrolytic procedures employ enzymes, acid, or alkali to rupture the cell walls. Cell autolysis is a well-known process wherein the yeast cell is subjected to digestion by its own enzymes.
Heretofore, it is believed that it has been difficult to extract nutrients from cells on a commercial scale, particularly from dead yeast cells, in light of certain drawbacks with the foregoing conventional methods. Mechanical rupture is attractive because the cell constituents are not contaminated with extraneous chemicals and additives. However, the costs associated with scaling-up and implementing such systems are considerable, as has been heretofore recognized. For instance, U.S. Pat. No. 5,756,135 issued to Seeley discusses some of the technological and economical challenges associated with commercial-scale production of a water insoluble yeast. Hydrolytic methods are more amenable to scale-up, but most such methods also have shortcomings such as high cost, long process time, or degradation/denaturation of specific nutrients.
Accordingly, most yeast cell hydrolyzates are produced commercially by autolysis. Yeast autolysis entails a slow reaction, however. An autolysis reaction requires an operating temperature that ranges from about 40° C. to 60° C., typically temperatures of 50° C.-55° C. At these or higher suitable temperatures, the reaction still requires a substantial period of time ranging from several hours to days to obtain a suitable degree of digestion. In an effort to accelerate the autolysis reaction, the prior art has taught to employ plasmolyzing agents, examples of which include organic solvents, salts and hydrolytic enzymes such as protease and lipases. Nonetheless, the autolysis reaction remains lengthy and commercially unwieldy.
A further drawback with autolysis is that the autolysis process is amenable only for use with living cells. Dead cells cannot be autolyzed. In recognition of this requirement, dedicated yeast manufacturers who desire to autolyze the yeast cells are required to take steps to preserve cell viability. In other industries where substantial quantities of live yeast are produced as a by-product, such as the brewing industry, live cells can be harvested economically and can be subjected to autolysis. However, certain industrial processes generate a substantial quantity of dead yeast by-product that cannot be subjected to autolysis. This is a particular problem in the production of distilled ethanol products, wherein the distillation process kills the yeast cells, thereby rendering the cells impossible to autolyze.
Accordingly, given the heretofore described drawbacks with mechanical and hydrolytic methods, it is very difficult to produce a cost-effective, high-volume yeast-derived feed or industrial product from such dead cells. In practice, the dead yeast cells themselves are sold as whole cells, typically into the ruminant animal feed markets.
It would be desirable to provide a method for disassociating yeast and other cells in a manner that allows for rupture of the walls of the cells to release the cell cytoplasm therefrom. It would be of particular benefit for such method to be applicable to dead cells in addition to live cells. Such method would find a particular applicability in the distilled ethanol industry, but would also be useful in connection with numerous other industries.